I am having the same problem on my Mac. It is running 10.8.5 MtLion. The problem occurred when I upgraded to iTunes 11.1 for iOS 7.
My configuration: 27″ iMac running on SSD with promise RAID for media files. Lots of RAM. I am using a B&W A7 AirPlay speaker connected directly to the Mac via optical cable.
I’ve learned that when the movie is playing in fullscreen mode, that’s when the sound starts cutting out. Actually it sounds like it’s skipping. Happens at a regular frequency of about 4 skips per second. If I take the movie out of full screen mode the sound cleans up immediately. Go back to Fullscreen mode and again skips. (I go in and out of full screen mode while the movie is playing). If I quit Digital Power Station, the sound cleans up, but looses the enhancements. When I restart it while the movie is playing, it will work for a few minutes then begin to skip again. Also, I believe the skipping gets more exaggerated the longer I let it run; the frequency stays constant, but it seems to be more pronounced. I hope this helps. Also, I find that it happens only in iTunes and it doesn’t matter whether I rip’d the files or purchased them from iTunes.